The present invention relates generally to light emitting elements and displays. More particularly, the present invention relates to an organic light emitting device that is controlled by a micro electromechanical system, a display constructed thereof, and a method of using and manufacturing the same.
Current flat panel liquid crystal display (LCD) technology uses a rigid substrate, such as glass, with thin film transistors (TFTs) corresponding to each display pixel. Due to the brittle nature of glass, the displays cannot withstand large amounts of mechanical vibration and/or pressure. This can be a major disadvantage for portable display applications, such as portable digital assistants (PDAs), which by their portable nature, can be exposed to a large amount of shock and abuse. Also, the properties of the LCD limit the operating temperature range of the device. Due to these limitations, other technologies, such as field emission displays (FEDs), in which electron emitters are controlled by cold cathodes to emit light, and organic light emitting devices (OLEDs) have emerged to overcome some of these limitations.
There have been several developments in FED technology. For example, U.S. Pat. No. 5,210,472 discloses a FED design which utilizes a matrix-addressable array of pointed, thin-film, cold emission cathodes in combination with a phosphor luminescent screen. The FED disclosed in U.S. Pat. No. 5,210,472 incorporates a column signal to activate a single conductive strip within the cathode grid, while a row signal activates a conductive strip within the emitter base electrode. At the intersection of both an activated column and an activated row, a grid-to-emitter voltage differential exists sufficient to induce a field emission, thereby causing illumination of the associated phosphor of a pixel on the phosphorescent screen. However, FEDs generally require tight tolerances for spacing between the emitters, the cold cathodes, and a phosphor screen in order for the electrons to pass through the gates and hit the phosphors. These requirements preclude the use of a flexible substrate, such as a polymer substrate, for the FED because flexure of the substrate would cause the spacing size to move out of acceptable tolerance.
An organic light emitting device (OLED) is typically a laminate formed on a substrate such as glass or silicon. A light-emitting layer of a luminescent organic solid is sandwiched between a cathode and an anode. The OLED may also include hole-injecting or electron-injecting layers. The light-emitting layer may be selected from a number of known fluorescent organic solids. The light-emitting layer may consist of multiple sublayers or a single blended layer. When a potential difference is applied across the device, electrons move from the cathode into the layer(s) of organic material. At the same time, holes move from the anode into the same organic light-emitting layer(s). When the holes and electrons meet in the layer(s) of organic material, they combine and produce photons. The wavelength of the photons depends on the material properties of the organic material in which the photons are generated. The color of light emitted from the OLED can be controlled by the selection of the organic material or by the selection of dopants, or by other techniques known in the art. In a typical OLED, either the anode or the cathode is transparent in order to allow the emitted light to pass through and out of the device.
U.S. Pat. No. 5,962,962 describes a basic organic light emitting device. The OLED has a structure in which an anode, an organic light emitting layer, and a cathode are consecutively laminated. Generally, electrical current flowing between the anode and cathode passes through points of the organic light emitting layer and causes it to luminesce. The electrode positioned on the surface through which light is emitted is formed of a transparent or semi-transparent film. The other electrode is formed of a specific thin metal film, which can be a metal or an alloy.
Known OLED displays use either a traditional passive or a thin film transistor (TFT) approach for creating and addressing a large number of 2-dimensional array pixels. Although the passive approach uses less power, it is difficult to maintain display uniformity over the entire display area. The TFT approach allows for better display uniformity but requires a high temperature fabrication process. This high temperature process requirement generally prevents any organic substrate from being used because of the breakdown in organic materials at high temperatures. As a result, most current OLEDs include a TFT formed separately on a glass substrate for the build-up process. Accordingly, conventional OLED display devices are not flexible and shock resistant. Another problem that arises when using TFTs as an addressing mechanism for pixels is that a certain amount of cross talk is almost unavoidable from each scanned row to the next scanned row.
It would be desirable to provide a resiliently flexible display that exhibits the advantages of organic light emitting device technology and avoids the disadvantages conventional devices, such as excessive cross talk.